The present application claims priority from European Patent Application No. 01300924.6, filed Feb. 1, 2001.
The present invention is directed to a method and apparatus for determining a zero gas flow state in a conduit in which bidirectional gas flow occurs. In a typical application, the invention may be used to determine zero gas flow in the patient limb of a ventilator breathing circuit. Such a zero gas flow state occurs in the transition from inspiration to expiration, and vice versa. The output signals obtained from flow sensors in the patient limb when the zero gas flow state is present are used for calibrating the sensors.
During anaesthesia and in intensive care, patients are commonly connected to a ventilator having a breathing circuit that provides breathing gases to/from the patient. The breathing circuit includes an inspiratory, or inhalation, limb, one end of which is connected to the ventilator. The other end of the inspiratory limb is connected to one arm of a Y-piece connector. A second arm of the Y-piece connector is connected to the patient limb of the breathing circuit for supplying breathing gases to the patient through a face mask, endotracheal tube, or other suitable appliance. The third arm of the Y-piece connector is connected to one end of the expiratory, or exhalation, limb of the breathing circuit. The other end of the expiratory limb is connected to the ventilator.
During inspiration, the expiration limb of the breathing circuit is closed by an expiration valve of the ventilator and, in spontaneous breathing, the underpressure generated by the patient""s lungs draws breathing gases from the inspiratory limb through the Y-piece connector to the patient limb and into the lungs. In artificial ventilation, the ventilator provides an overpressure in the breathing circuit that supplies the breathing gases into the lungs of the patient. During expiration, the expiration valve in the ventilator or breathing circuit is opened and the contraction of the patient""s rib cage forces breathing gases through the patient limb and Y-piece connector into the expiration limb of the breathing circuit for discharge from the ventilator. The process is repeated on the next breath.
During each breath, bidirectional gas flow occurs in the patient limb of the breathing circuit. The breathing gases flow first in one direction into the lungs of the patient during the inspiratory phase of the respiratory cycle. Thereafter, at least an instant of zero gas flow occurs in the patient limb during a transition between inspiration and expiration. During expiration phase of the respiratory cycle, breathing gases flow in the opposite direction through the patient limb of the breathing circuit from the patient to the ventilator.
Gas flow rates in the breathing circuit can vary over a relatively wide range in the course of respiration, among patients of different ages, and with differing patient pulmonary conditions.
In one type of flow sensor commonly used to measure breathing gas flow rates, a flow restricting element is placed in the patient limb or other conduit. A pressure drop proportional to the gas flow rate is generated across the flow restrictor. Gas pressure sensing ports may be provided upstream and downstream of the flow restricting element and the pressure drop across the restrictor is measured by a differential pressure sensor. The differential pressure sensor can also determine, from the relative magnitudes of the pressures used in measuring the pressure drop, the direction of gas flow in the conduit.
Depending on the configuration of the flow sensor, the relationship of the magnitude of the pressure drop to gas flow rate may vary from that expressed by a linear relationship to one in which the pressure drop varies exponentially, for example, as the square of the gas flow rate.
While use of a flow sensor with a linear gas flow rate-output signal relationship simplifies gas flow sensing, it requires laminar flow conditions in the restrictor. This, in turn, necessitates the use of a plurality of finely structured flow channels in the flow restrictor to obtain the pressure drop. For applications such as measuring the flow rate of breathing gases, the finely structured flow channels may become blocked by mucus or other excretions from the lungs of the patient.
These difficulties have led to the use of flow restrictors in which the output signal varies as the square or some other exponent of the gas flow rate. While such sensors work well at high gas flows, the output signal is very small at low flows. This makes highly accurate gas flow measurements at low gas flow rates difficult. To obtain highly accurate measurements at low flows, it is necessary to obtain the sensor output signal at a zero gas flow condition in order to calibrate the sensor to remove errors that adversely effect low flow measurement.
To obtain the flow sensor output signal at zero gas flow conditions, the differential ports of the sensor can be temporarily short circuited, as for example, by means of a solenoid valve. However, during such a procedure, the measured gas flow rate output from the sensor is not available for patient monitoring purposes. This may limit the frequency with which calibration procedures can be carried out. But, errors or drift may arise in the sensor if the sensor calibrated too infrequently.
Also, the solenoid valve adds to the bulk of the gas flow sensor. Since the gas flow sensor is located in the patient limb, this can be a serious problem as the space around the patient""s head may be crowded with other equipment. While elements of the flow sensor can be located remotely from the patient limb gas flow path, to transport the signals from the gas ports in the flow restrictor to the remote elements requires a double lumen, relatively large dimension tubing. This does not totally relieve congestion at the patient.
Another type of gas flow sensor is the so-called xe2x80x9chot wirexe2x80x9d anemometer. In such an anemometer, a thin resistive wire is placed across the gas flow conduit so that the gas flows over the wire. The wire forms one arm of a Wheatstone bridge circuit. Energization of the bridge circuit passes current through the resistive wire increasing its temperature and causing it to become a xe2x80x9chot wire.xe2x80x9d The resistance of the wire is proportional to its temperature. The flow of the gas past the hot wire carries off heat from the wire, altering its resistance in accordance with the amount of gas flow. To determine the amount of gas flow, the amount of current necessary to restore the hot wire to the original temperature may be used as an indication of the amount of gas flow. Or, the energization of the bridge may be kept constant, and the alteration in the resistance of the wire as its temperature is reduced, as reflected in the resulting imbalance in the bridge, can be used to determine the gas flow rate. While the anemometer has been described as using a wire, it can also be formed using a film.
However, in its simplest embodiment, a hot wire anemometer is not a direction sensitive flow sensor. Further, a major problem with such a flow sensor is that the only way to calibrate the hot wire anemometer at the zero flow condition is to stop the flow of gas in the conduit. In applications such as a breathing circuit for a patient, this creates significant, practical problems.
It is, therefore, an object of the present invention to provide a method and apparatus by which the zero gas flow state in a bidirectional gas flow conduit can be determined using a flow sensing device having a pair of flow sensors. Determination of the zero gas flow state enables the device to be calibrated using zero gas flow offsets, thereby to provide accurate gas flow rate measurements over a wide range of flow rates, including low flow rates.
The present invention can be used in a patient breathing circuit and can accomplish the foregoing and other objects without altering the operation of the breathing circuit and without the need for the additional components, such as solenoid valves, closely associated with the flow sensors or patient limb of the breathing circuit, that have heretofore rendered flow sensor devices more difficult to use.
Briefly, in the present invention, a first signal responsive to the gas flow in conduit is obtained using a bidirectional flow sensor. The first signal has one polarity when the gas flow is in one direction in the conduit and the opposite polarity when the gas flow is in the opposite direction. A second gas flow signal is obtained using a uni-directional flow sensor, the output signal of which has the same polarity for either direction of gas flow in the conduit.
The derivatives of the first and second signals are obtained at successive points in time during gas flow in the conduit, for example that occurring during inspiration or expiration by the patient. The derivatives describe the instantaneous changes of the signals with respect to time. Whether the changes increase or decrease the output signals is indicated by the signs of the derivatives, i.e., as plus or minus signs.
During an inspiration or expiration phase of the respiratory cycle, the signs of the derivatives of the first and second signals will be present in a first predetermined combination of signs.
At the end of an inspiration or expiration, there will be a period of at least momentary zero gas flow during the transition to the subsequent phase of the respiratory cycle. The output signals from the sensors will ideally also be zero but, as a practical matter, will usually provide some output signal.
As respiration continues, the signs of the derivatives of the first and second signals are examined to detect changes in the signs. If, for example, the sign of the first signal derivative (bi-directional flow sensor) is unchanged, whereas the sign of the second signal derivative (unidirectional flow sensor) has changed, this indicates that a zero gas flow condition has occurred at the point in time when the sign of the second signal derivative changed. Alternatively, the zero gas flow may produce a zero slope derivative, i.e., one with no sign in one or both the signals and the period during which this occurs may be taken as one of zero gas flow.
The output signals from the sensors obtained in the zero gas flow state may be used to calibrate the flow sensors to obtain accurate gas flow rate measurements from the sensors.
Noise in the output signals may be filtered out or appropriate signal thresholds employed to facilitate determining the zero gas flow state.
Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the accompanying drawing.